Dear Kamau,
x
Thanks for choosing YouMail! Due to recent T-Mobile policy changes prepaid and flex-pay accounts no longer support conditional call forwarding. This would be why the call is showing “barred” or “not allowed”. Without this feature you will be unable to send your unanswered calls to YouMail. We apologize for the inconvenience however T-Mobile policies are beyond our control. If you would like more information, please call tech support at (800)374-0013 Mon-Fri 9am-5pm PST for further assistance. Thanks!

This is a copy of the email I got, just in case anyone else has the same question.